Various industrial applications require the conversion of electric power from direct current (DC) voltage to alternating current (AC) voltage and vice versa. This can be achieved using Voltage Source Inverter (VSI) circuits and Power Factor Correction (PFC) circuits, respectively. Modern power electronics converters require high performance, such as high efficiency and high power density, and low cost. For some applications, the requirements are even higher in order to satisfy industrial standards, such as is the case with leakage current limits. However, most power converter circuits that are designed to try to minimize leakage current consequently reduce efficiency and power density.
In PFC circuits, the power factor of the AC flowing through the circuit should be corrected to be as close to 1 such that the real power delivered to the circuit is the same as the apparent power before being converted to DC. Correction of the power factor is typically achieved using a passive network of capacitors or inductors. The AC voltage is converted to the DC voltage using some sort of rectifier, however the DC output that is generated typically comprises pulses of current because the AC is sinusoidal, which accordingly may be smoothed out using a filter (usually some sort of capacitor arrangement).
PFC circuits are used in a wide range of applications, including but not limited to motor drives, electric vehicle chargers, electronic ballasts, uninterrupted power supply systems, etc. However, several problems exist when these circuits are implemented. A first problem is a low efficiency problem due to high losses in the conversion of the voltage. This is because in a power supply unit there are usually two power stages, the first being a PFC stage to shape the input current to be sinusoidal to meet industrial standards and to step up the grid voltage (e.g. 120V) to be higher than the peak voltage of the grid (e.g. 200V), and the second stage comprising a buck-converter to step down the voltage (e.g. 48V). A second problem is a common mode voltage issue, which is a common problem for bridgeless converters (used so that the output has the same polarity as the input). Conventional bridgeless converters create high voltage jumping between a negative port of the DC link and system ground, which leads to high leakage current that fails to meet industrial standards. Attempts have been made to try to correct these issues, but often require a large number of semiconductors in the current path leading to high conduction losses and costs, and/or require large input filters.
Broadly, VSI circuits perform the opposite of PFC circuits in that they are converting a DC voltage to an AC voltage. Since the voltage is being converted from DC, the power factor should already be equal to 1 and thus there is no power factor correction required in AC. VSI circuits are used in a wide range of applications, including but not limited to: photovoltaic (PV) inverters where the variable DC output of a solar PV panel is converted into AC to be fed into a commercial electric grid or for use by an off-grid network, fuel cell inverters which perform similar function but with fuel cells, other grid-connected applications, etc.
However, several problems exist when these circuits are implemented. Similar to PFC circuits, there are low efficiency problems and common mode voltage issues. With regards to a PV inverter for example, there are typically two power stages—the first being a DC-DC boost converter for Maximum Power Point Tracking (MPPT) to determine the maximum power output from the PV power and to maintain the DC voltage link at a high enough value for the second power stage, which is the VSI comprising a buck-type DC-AC inverter and which shapes the output current as a sine wave to meet industrial standards. The first stage requires a boost converter to bring up the voltage from the PV panels to the DC link capacitor bank. This approach gives an unimpressive efficiency for the whole converter of maximum 97% for a 1.5 kW system, due to the two power conversion stages.
Again using a PV inverter as an example, the common mode voltage issue is typical for transformerless PV inverters because there is high voltage jumping between a negative port of PV panels and the system ground, this leads to a shorter lifetime of the PV panels and creates electromagnetic interference noise to the grids, as well as potentially poses a safety issue for users. Thus there are regulations to limit the value of the leakage current of a VSI (for example, in Europe: DIN V VDE V 0126-1-1, which limits the leakage current to 300 mA).
Conventionally, using bi-polar switching was a solution to the leakage current issue, however this creates increased switching losses and increases the size of grid inductors in order to achieve small current ripples. Other attempts have been made to solve these problems but produce negative effects such as increased conduction losses due to a large number of semiconductors.
Accordingly, an improved bridgeless buck-boost electrical power converter circuit remains highly desirable.
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.